154 THE OSTEOLOGY OF THE REPTILES 



ischium only, and, like that of the Chelonia, is directed upward and 

 backward. The pubes and ischia, like the coracoids, are very broad 

 and flat, secondarily plate-like, meeting in a more or less horizontal 

 symphysis. There is no pubic foramen, and usually the large pubo- 

 ischiatic vacuity is broadly connected across the median line — prob- 

 ably separated by a ligament in life. In some genera, however, 

 S thenar osaur us or Thaumatosaurus, for instance, the two bones are 

 secondarily broadly united at their symphyses, producing a false 

 thyroid foramen with which the obturator foramen is confluent, as in 

 mammals. The ischia are triangular or "hatchet-shaped," elongated 

 in the short-necked forms, short in the long-necked. 



The pelvis of the Chelonia (Fig. 127), like the pectoral girdle, has 

 been modified by its peculiar relations to the carapace and plastron. 

 There is a large pubo-ischiatic vacuity, often divided in the middle 

 by a cartilaginous septum, but broadly ossified in the land tortoises, 

 as in the plesiosaurian Sthenarosaurus. As in the plesiosaurs, there is 

 no separate pubic foramen or notch, rarely absent in reptiles. 



The ilium, like that of the plesiosaurs, is elongate and is directed 

 upward and backward to the firm sacrum. The pubis is larger than 

 the ischium and has a stout tuberosity which rests upon the plastron, 

 or, in the Pleurodira, is coossified with it. 



Usually in crawling reptiles (Figs. 114-118 a) there is no, or only a 

 small, preacetabular process to the ilium, but always a postacetabu- 

 lar one. In upright- walking animals the preacetabular process is al- 

 ways well developed, sometimes at the entire expense of the postace- 

 tabular process. It is unusually long in the Anomodontia (Figs. 

 120 c, 119), Ceratopsia (Fig. 122 c, e), and Pterosauria (Fig. 118 d), 

 where it is supported by the united or contiguous diapophyses of the 

 lumbar vertebrae, false sacral vertebrae. The ilium is more or less 

 helmet-shaped in the Saurischia (Fig. 122 a, b) as also in some Coty- 

 losauria, Therapsida (Cynognathus), and Theromorpha (Casea) — all 

 such forms have short toes; possibly it is due to the greater expansion 

 of the gluteal muscles. 



The evolution of the reptilian pelvis has been, as we have seen, 

 from the primitive closed and plate-like type, by the progressive de- 

 velopment of a vacuity between the ischia and pubes, by the elonga- 

 tion of the anterior process of the ilium, and by its closer union with 

 additional true sacral or lumbar vertebrae. 



